In a background art, as a flow rate sensor which is installed at a suction air passage of an internal combustion engine of an automobile or the like for measuring a suction air amount, a thermal type one is mainly used. This is because a thermal type flow rate sensor can directly measure a mass flow rate.
In recent years, low fuel consumption and exhaust gas regulation are becoming severe from a view point of global environment protection. Therefore, in a flow rate sensor which measures a suction air amount, there are manifested needs for high accuracy, backflow detection, dynamic range expansion, and suchlike.
In a thermal type flow rate sensor which deals with such needs, in recent years, attention is paid to fabricate a sensing element which measures a flow rate on a semiconductor substrate of silicon or the like by using a semiconductor microfabrication technology. Because a sensing element of this kind can comparatively easily be mass-produced, and therefore, the sensing element is excellent in economy, can be downsized and can be driven by low power consumption. As such a flow rate sensor, there is a flow rate sensor described in Patent Literature 1.
In the case of the flow rate sensor described in Patent Literature 1, the sensing element is formed with a sensing resistor on a silicon substrate via an insulating layer, and formed with a thin film portion (diaphragm portion) by removing a portion of the silicon substrate in order to thermally insulate the resistor. A heat generating resistor can be formed by arranging a resistor which is driven as a heater at the diaphragm portion. In detecting a flow rate, there is adopted a temperature difference system in which temperature sensing resistors are formed on an upstream side and on a downstream side of an air flow by interposing the heat generating resistor, and a flow rate and a direction are detected based on a difference between temperatures of the temperature sensing resistors that are arranged on the upstream side and on the downstream side.